Each agent maintains a database of keyframes. To populate the database, we follow the same approach proposed for PTAM [1]}. Namely, every time the ratio between the baseline \(b\) , that is the distance between the previous keyframe and the current camera pose, and the average depth \(\bar{z}\)  of the tracked features is higher than a threshold that is usually around \(10-20\%\) , a new keyframe is defined.
